For use in multiplex transmission of data including a video signal, an audio signal and a control signal between video apparatuses such as a television camera (hereinafter referred to as camera unit) and a camera control apparatus (hereinafter referred to as camera control unit), there is known an apparatus which subjects data including the video signal, the audio signal and the control signal to frequency division multiplexing and transmits the multiplexed signals over a triple coaxial cable known as a Triax cable (hereinafter referred to as transmission cable).
In Triax transmission, bidirectional transmission of the video signal, the audio signal, the control signal and the like subjected to analog frequency division multiplexing has constituted the main stream. Such analog processing is susceptible to the influences of the characteristics of the transmission cable used and those of the filter used for frequency division, often resulting in deterioration in the characteristics of the video and audio signals received in the camera unit or the camera control unit.
With a view to solving this problem, the present applicants proposed a multiplex transmission method for digital video signals and an apparatus therefor, disclosed in the Japanese Patents No. 390509 and No. 3194510 for instance. According to these patents, each of the video devices provided at the two ends of the transmission path, video signals and audio signals are digitized, subjected to time division multiplexing and compressed on the time axis to generate transmission signals consisting of repeated alternation of signal periods and non-signal periods. The transmission signals are sent from a video device connected to one end of the transmission path, and transmission signals are sent from another video device connected to the other end of the path during the non-signal periods, thereby enabling bidirectional transmission to be achieved over a single transmission path. The apparatuses embodying this principle, disclosed in the patents, are already available for practical use.
One example of conventional multiplex transmission apparatus for digital video signals will be described with reference to FIG. 7. Referring to FIG. 7, first, in a camera unit 701, a video signal S1 and an audio signal A1 from a camera 702 and a control signal C1 from a CPU 710 for controlling the camera 702 are converted into digital signals by an A/D converter (not shown), multiplexed by a time division multiplex circuit 703, converted into serial data by a parallel/serial converter 704, and supplied to an amplifier 705. The output of the amplifier 705 is transmitted to a camera control unit 720 via a cable 713 (which generally refers to the transmission path). In the camera control unit 720, the time division-multiplexed signals from the cable 713 are amplified by an amplifier 721, converted into parallel data by a serial/parallel converter 723 after being compensated for cable attenuation by an equalizer 722, and separated into a video signal S1, an audio signal A1 and a control signal C1 by a separator 724.
Similarly in the camera control unit 720, a digitized video signal S2 and a digitized audio signal A2 from a CCU 726 and a control signal C2 from a CPU 725 for controlling the CCU are inputted to a time division multiplex circuit 728. A reference video phase signal TRS (time reference signal) from a reference video phase signal generator 727 is inputted to the time division multiplex circuit 728, wherein these and other signals are multiplexed, converted into serial data by a parallel/serial converter 729, and transmitted to the camera unit 701 via an amplifier 730 and the cable 713. In the camera unit 701, time division multiplexed signals from the cable 713 are amplified by an amplifier 706, compensated for cable attenuation by an equalizer 707, converted into parallel data by a serial/parallel converter 708, and separated into the video signal S2, the audio signal A2 and the control signal C2 by a separator 709 to be supplied to the camera 702 and the CPU 710. The reference video phase signal TRS is a 10-bit digital signal consisting of a combination of (3FF) and (000), so composed that it can be identified as a TRS upon receipt of this signal.
Further in the camera unit 701, the reference video phase signal TRS is detected by a reference video phase signal detector 712, a reference video phase signal TRS is generated by a reference video phase signal generator 711 for the camera unit 701, and the latter is applied to the camera 702. Thus, the reference video phase signal TRS from the reference video phase signal generator 727 of the camera control unit 720 is detected by the camera unit 701, a reference video phase signal TRS is generated in synchronism with it, and the camera 702 is driven in synchronism with this reference video phase signal TRS.
Then, in the digital video signal multiplex transmission apparatus described above, if the various signals are simply multiplexed and serialized, the band required for signal transmission will widen. Accordingly, there are disadvantages of increased deterioration in signal characteristics due to the cable loss on the transmission path and a reduced transmittable length. In other words, though there is no deterioration, which would arise in analog transmission, within the distance in which digital signals can be transmitted (the length of the transmission path), the digital signals cannot be normally reproduced beyond the distance of digital transmittability, resulting in a state in which normal transmission is impossible.
More specifically, for instance D1 signals according to the digital signal standards instituted by SMPTE (Society of Motion Picture and Television Engineers (U.S.)), the quantity of signals that can be transmitted is 270 Mbps. D1 signals are used as main signals from the camera unit 701 to the camera control unit 720 and, the return signals R1 equivalent to 90 Mbps are used in the other way around, namely from the camera control unit 720 to the camera unit 701, because poorer video quality poses no problem in this direction. In this setting, the combined transmission quantity of the main signals D1 and the return signals R1 will be 360 Mbps. Eventually, it is required to perform bidirectional transmission in a transmission band of 360 Mbps.
On the other hand, the value of attenuation by the cable 713 increases with a rise in frequency. The case of the commonly used Triax cable, for instance, is as shown in Table 1 below.
TABLE 1Frequency ofAttenuationCable lengthtransmitted signalvalue100m360MHz−10 dB1000m36MHz−28 dB
Now, attenuated serial digital signals can be restored by the equalizers 707 and 722, which are compensators, but their restoration has its own limit. Thus, where the frequency of transmitted signals is high, such as 360 MHz, the value of attenuation of around −40 dB is the limit of restoration. In a simple calculation, where the frequency of transmitted signals is 360 MHz as in the case of Table 1:−40 dB/(−10 dB/100 m)=400 m  (1)Thus, the maximum cable length that permits transmission is 400 m. Supposing the limit to compensation is supposed to be the same where the frequency of transmitted signals is 36 MHz:−40 dB/(−28 dB/1000 m)=1429 m  (2)Thus, the maximum cable length that permits transmission is about 1.5 km.
Meanwhile, camera systems using a Triax cable are used for many different purposes. In a broadcasting station, for instance, they are more often used in studios, where the cable length is usually not more than 100 m, short enough to meet the 400 m requirement of Equation (1) and posing no particular problem as long as they are used in studios. However, when they are used outdoors for telecasting a baseball game or a golf tournament for instance, the distance between the camera unit 701 and the camera control unit 720 is in most cases more than 1 km. Though this long distance invites no deterioration where digital signals are transmitted, there is a problem of a reduction in maximum permissible cable length.